The present invention relates to the technical field of receiving and detecting or discriminating information or signals transmitted in an optic link in particular an infrared link. More precisely, the present invention relates to an electronic circuit for receiving and detecting or discriminating modulating light of a specific modulation frequency or within a specific modulation frequency range.
In the present context, the expressions detecting and discriminating are to be conscrued generic terms which generally describe the technical phenomenon of evaluating an information signal which is received for determining whether or not the information signal is relevant or constitutes an information signal intended to be received and further processed by the receiver in question.
In numerous applications, an electronic circuit is used for receiving and detecting or discriminating modulated light including information or signals to be received and detected by the electronic circuit for controlling or displaying electronic equipment connected to the electronic circuit. Examples of applications or electronic circuits of this kind are among others wireless audio transmission systems, remote controllers for audio equipment or similar applications including door controllers, car blocking systems, data communication linkages etc. A particular application of electronic circuits of this kind is within the field of transmitting information from a master station or a back office computer to electronic price displays of the general type described in:
U.S. Pat. Nos 3,400,270, 3,867,628, 4,002,886, 4,201,471, 4,521,677, 4,717,913, 4,731,529, 4,766,295, 4,847,483, 4,879,756, 4,888,709, 4,937,586, 4,945,225, 4,962,466, 5,019,811, 5,111,196, 5,194,860, 5,241,657, 5,382,824, 5,406,271, EP 0 307 911, WO 86/02477, WO 91/08539, SE 461,690, DE 2,432,402, DE 3,823,007, DE 38 73 231, and GB 2,171,512.
Reference is made to the above patent applications and patents and the above US-patents are hereby incorporated in the present specification by reference. In Applicant's pending European patent application No. 92907631.3 and corresponding pending U.S. patent application Ser. No. 122,474, a specific verification technique is described according to which an electronic price display verifies the authenticity of the price display in question and the proper receipt of the intentional information in relation to the master station through the transmission of a verification signal to the master station. Generally within the above technical fields and in particular within the technical field of transmitting information through an optic link, in particular an infrared link from the master station to an electronic display, specific concerns have to be made in order to ensure proper operation of the entire apparatus or system. Firstly, the power consumption of the electronic price display has to be kept fairly low in order to reduce the power consumption of the self-contained electronic price display. Secondly, due to numerous radiating sources such as the day light or sun light, the radiation from electric light sources including incandescent lamps and fluorescence lamps the electronic circuit of the electronic price display has to be able to discriminate between the background radiation and the modulated light to be received and detected by the electronic circuit. Mains induced background light fluctuations may further influence the receipt of the information by the electronic circuit and has to be clearly discriminated in relation to the intentional modulated light to be received by the electronic circuit. Furthermore, It has to be realized that the background intensity of the light most often is several decades larger than the intensity of the modulated light to be received and detected by the electronic circuit.
Commonly, the electronic circuit includes a photodiode constituting the optic converter element receiving the modulated light In particular the modulated infrared radiation and converts the optic information signal into an information signal. In an electronic equivalence diagramme, the photodiode may be considered constituting a current generator connected in parallel with a diode. Provided the photodiode is exposed to light, the current through the current generator produces a specific voltage drop across the diode in the conducting direction of the diode. The voltage is built up to a certain level corresponding to the diode current derived therefrom being equivalent to the current generated by the current generator less the current derived to the peripheral circuit i.e. the electronic components connected to the photodiode.
In order to reduce the current shunted through the internal diode it is a common practise to bias the photodiode in the reverse direction and measure the current conducted through the biasing circuit by the current generator. As the diode is biased in the reverse direction, the internal diode does not shunt the photo current but gives rise to a reverse current (dark current) which is insignificant in relation to most communication purposes. The dark current may be reduced to approximately 0 mA by providing a biasing circuit maintaining a voltage of approx. 0 V across the photodiode, e.g. by using an operational amplifier in an active current configuration.
However, the prior art biasing circuit reducing a 0 mA dark current through the generation of a 0 V voltage across the photodiode causes the current generated by the diode through the exposure to light to be conducted through the current supply of the apparatus and give origin to a power consumption proportional to the incident light. As the photodiode may generate a current of the order of 0.1 mA through the exposure to background light a corresponding constant current has to be produced by the power supply of the apparatus. In battery powered equipment, a constant current consumption of the order of 0.1 mA is in most applications unacceptable.